Just to report back in on the outcome of the latest trip to Nikon for my D300. They had to replace a battery contact due to a failed solder joint. So not a firmware issue nor a sporadic problem I would just have to put up with.
Hopefully I will not be experiencing this particular problem again. I like my D300 but am anxiously awaiting the D400 if it has improved high ISO capabilities. Wouldn't mind 16MP either.